1. Field of the Invention
The present invention relates to a surface acoustic wave device including a piezoelectric substrate, an IDT electrode, and a SiO2 film covering the piezoelectric substrate and the IDT electrode. In particular, the present invention relates to a surface acoustic wave device including an IDT electrode formed by filling grooves provided in the upper surface of a piezoelectric substrate with a metal.
2. Description of the Related Art
Duplexers (DPX) and RF filters, which are used for mobile communication systems such as mobile phones, are required to have broadband characteristics and good temperature characteristics. Surface acoustic wave devices have been commonly used as DPXs and RF filters.
Surface acoustic wave devices used in such applications typically include a piezoelectric substrate made of LiTaO3, LiNbO3, or other suitable material and an IDT electrode thereon. LiTaO3 and LiNbO3 have negative values of a temperature coefficient of resonant frequency (TCF). To improve such temperature characteristics, one technique is known in which a SiO2 film having a positive temperature coefficient of resonant frequency is arranged on a piezoelectric substrate to cover an IDT electrode.
However, this technique produces a height difference of the SiO2 film between a portion in which fingers of an IDT electrode are provided and a portion in which the fingers of the IDT electrode are not provided. The height difference causes insertion loss to increase.
To solve this problem, Japanese Unexamined Patent Application Publication No. 2004-112748 discloses a method including forming a first insulator layer, which has substantially the same film thickness as an IDT electrode, between the IDT electrode fingers, and forming a SiO2 film to cover the IDT electrode and the first insulator layer. In this method, since the SiO2 film is formed on a substantially flat base, the SiO2 film has a substantially flat surface. The IDT electrode of the surface acoustic wave device in Japanese Unexamined Patent Application Publication No. 2004-112748 is made of a metal having a higher density than Al or an alloy primarily including the metal, or a multilayer film including a metal having a higher density than Al or an alloy primarily including the metal and another metal. The IDT electrode has a density at least about 1.5 times that of the first insulator layer.
Since the electrode of the surface acoustic wave device in Japanese Unexamined Patent Application Publication No. 2004-112748 primarily includes a metal that is heavier than Al, large variations tend to occur in the velocity and the frequency of acoustic waves which depend on variations in the thickness of the electrode. In contrast, an electrode made of Al has a very low reflection coefficient and cannot exhibit characteristics sufficiently good for surface acoustic wave resonators and surface acoustic wave filters.
To overcome such a problem, WO2006/011417A1 discloses a configuration including a piezoelectric substrate made of LiTaO3 or LiNbO3 having a plurality of grooves provided in the upper surface and an IDT electrode formed by filling the grooves with a metal, such as Al. A surface acoustic wave device of WO2006/011417A1 includes an IDT electrode formed by filling grooves with metal and a SiO2 film covering the IDT electrode. WO2006/011417A12 discloses that the SiO2 film is formed on a substantially flat base, and thus, the SiO2 film has a substantially flat surface.
WO2006/011417A1 also discloses that LiTaO3 substrates with Euler angles and LiNbO3 substrates with Euler angles (0°, 85° to 120°, 0°), (0°, 125° to 141°, 0°), (0°, 145° to 164°, 0°), or (0°, 160° to 180°, 0°) are preferably used; and LiNbO3 substrates with Euler angles (0°, 90° to 110°, 0°), (0°, 125° to 136°, 0°), (0° 149° to 159°, 0°), or (0°, 165° to 175°, 0°) are more preferably used.
A configuration intended to have improved temperature characteristics of resonant frequency that is formed by disposing a SiO2 film on an IDT electrode made of Al provides a low reflection coefficient as described above and tends to have insufficient characteristics. In contrast, since the configuration in WO2006/011417A1 includes an IDT electrode formed by filling grooves provided in a piezoelectric substrate with Al, the electrode has a sufficiently high reflection coefficient. The configuration also includes a SiO2 film and thus has improved temperature characteristics of resonant frequency. The SiO2 film has a substantially flat surface, and thus, the insertion loss does not significantly increase.
However, an inventor of the present invention has discovered that, when a response of a Rayleigh wave is utilized in the surface acoustic wave device including a LiNbO3 substrate in WO2006/011417A1, a large spurious response occurs near an attenuation pole of a main response being used. Thus, the spurious response may potentially degrade the filter characteristics and the resonance characteristics.